The present invention generally relates to rechargeable batteries, and particularly relates to battery charging methods and apparatus that offer improved charge current sensing accuracy.
Rechargeable batteries appear in a growing range of electronic devices. The use of rechargeable batteries is particularly common in portable electronics, such as cell phones, Portable Digital Assistants (PDAs), pocket and notebook computers, Global Positioning System (GPS) receivers, etc. No one rechargeable battery type finds universal usage across this diverse range of devices, as each battery type offers its own set of tradeoffs regarding performance, size and cost.
For example, types of rechargeable batteries include, but are not limited to, lead-acid cells, nickelcadmium cells, nickel/metal hydride cells, sodium/sulfur cells, nickel/sodium cells, lithium ion cells, lithium polymer, manganese-titanium (lithium) cells, nickel zinc cells, and iron nickel cells. Each of these battery chemistries offers its own mix of advantages and disadvantages regarding size, energy density (volumetric or gravimetric), cost, cell voltage, cell resistance, safety, toxicity, etc.
Despite such differences, some charging algorithms find broad applicability across a wide range of battery chemistries. As an example, the constant-current/constant-voltage (CC/CV) charging algorithm is adaptable to many different types of battery chemistries and, therefore, finds wide usage in a variety of battery charging products. With the CC/CV charging algorithm, a discharged battery is charged at a constant current until its cell voltage rises to a defined threshold voltage, e.g., the battery""s xe2x80x9cfloat voltage,xe2x80x9d at which point the charging control is switched to constant/voltage to charge the remaining capacity of the battery without exceeding the voltage limit of the battery. Thus, the CC/CV charging algorithm initially relies on current feedback charging control and then switches over to voltage-feedback control once the battery-under-charge reaches its float voltage.
As a matter of convenience, the charging current during the CC phase of recharge should be as high as possible within recommended limits because higher charging currents equate to lower recharge times. Battery manufacturers often rate battery capacity in terms of a given battery""s xe2x80x9cC rating,xe2x80x9d which is a scaling unit for the battery""s charge and discharge currents. Charging or discharging the battery at rates beyond the xe2x80x9cCxe2x80x9d rating exceeds the safe rating of the battery. For example, a charge current of 1000 mAh (1 C) will charge a 1000 mAh battery in about an hour. Thus, charging current during the CC phase of recharging may be set according to the manufacturer""s recommended C rate limit.
Proper C rate based charging requires relatively accurate current sensing to ensure that the CC phase of charging actually is regulated to the recommended C rating of the battery. In the above example, the 1 C charging current is 1 Amp and thus the current sensing used to regulate the charging current must sense current in the 1 A range with relatively good accuracy.
Most simple and economical current sensing techniques rely on Ohm""s Law (V=IR), and thus measure current by sensing a current-induced voltage drop across a resistor or other series impedance element. For efficiency and low voltage dropout, such sense resistors typically are sized for a particular current range of interest. For example, a 100 mxcexa9 sense resistor provides a very usable 100 mV sense voltage at 1 A of charging current. Obviously, as the charging current falls, so too does the sense voltage, and therein lies one of the many challenges faced by designers of battery charging systems. That is, the sense voltage decreases with decreasing charge current and circuit error sources, such as sense amplifier voltage offsets, become increasingly significant obstacles to accurate charge current measurement.
Further compounding these design challenges, not all charging environments offer the ability to charge at a battery""s recommended C rate. For example, many battery-powered devices interface to Personal Computers (PCs) and the like via Universal Serial Bus (USB) connections. Portable music players, such as those based on the popular MP3 digital audio format, are just one example of such devices. Regardless, the USB standard defines low-power devices as those requiring less than 100 mA, and high power devices as those requiring up to 500 mA. Some USB ports support both low power and high power, and thus offer attached devices the ability to draw charging currents up to the high power limit of 500 mA. However, some USB ports support only low power devices and thus limit charging current to 100 mA. Ideally, a rechargeable USB device would operate with accurate charge current sensing regardless of the type of USB port to which it is attached.
Further charging scenario variations create additional challenges to reliable and accurate current sensing. For example, some types of battery technologies are incompatible with so-called xe2x80x9ctricklexe2x80x9d charges, i.e., continuous low current into the battery after it reaches its float voltage. Because of the inability of conventional charging systems to accurately sense very low levels of battery current, it is a common practice to include a transistor switch or other isolation device to xe2x80x9cdisconnectxe2x80x9d the battery from the charging circuit after reaching an end-of-charge condition. The addition of the extra switching element adds undesirable expense and size to the charging circuit.
For these and other reasons, an ideal battery charging circuit would offer highly accurate current sensing over a wide range of charging currents. Accurate current sensing over a wide dynamic range would thus allow reliable and safe charging in various modes (e.g., low power and high power), would permit accurate end-of-charge current sensing, and would permit regulation down to an effectively xe2x80x9czeroxe2x80x9d charge current. That latter capability would eliminate the requirement for battery isolation switches as the charging circuit itself could regulate current into the battery essentially down to zero.
The present invention comprises a system and method to charge batteries based on accurately sensing battery charging currents over a wide dynamic range. In an exemplary embodiment, a battery charging circuit includes a sense circuit that includes a time-averaging amplifier circuit to accurately sense the charging current of a battery under charge. An exemplary time-averaging amplifier circuit comprises a polarity-switched amplifier that periodically switches amplifier signal polarities to null amplifier-offset errors from the sensed charging current.
For example, in at least one embodiment, the sense circuit generates the sense signal as a voltage that is proportional to the charging current, wherein the polarity-switched amplifier is used as a differential sensing amplifier that controls the sense signal. An adjustable element, such as a user-set resistor, may be used to provide a desired scaling between the sense signal and the battery charging current magnitude. In one polarity configuration, the polarity-switched amplifier""s offset errors add to the sense signal and in the opposite polarity configuration those same offset errors subtract from the sense signal. Thus, by periodically switching between these polarity configurations, the amplifier""s offset errors effectively are averaged out of the sense signal.
In at least one embodiment, the polarity-switched amplifier includes a differential amplifier having switched amplifier input and output connections. In an exemplary embodiment, one input switch selectively couples a current sensor, such as a series resistor disposed in the charging current path, to the non-inverting and inverting inputs of the amplifier, and another input switch selectively couples a sense feedback signal to the inverting and the non-inverting inputs of the amplifier. Similarly, an output switch selectively couples an output terminal of the amplifier to the positive and negative outputs of the differential amplifier. Each switch reverses its connections responsive to a polarity switching signal; thus, by driving the switches with a periodic clock signal, the amplifier""s input and output connections are periodically reversed, which nulls, or at least substantially reduces, amplifier offset errors.
Substantially removing offset errors from the sense signal eliminates a principal error term associated with charge current sensing and permits the same sense circuit to be used over a wide range of battery charging circuits. Among the many advantages yielded by that capability is the opportunity to use a single sense resistor for detecting a wide range of charging currents. That is, with the effective elimination of amplifier offset errors, sense voltages in the range of the sense amplifier""s input and output offset voltages can be detected. As such, the same sense circuit may be used to accurately detect high and low magnitude charging currents, as well as trickle charge currents and end-of-charge currents.
For example, in an exemplary embodiment, a battery charging circuit according to the present invention includes a charging control circuit coupled to the sense circuit. An exemplary charging control circuit comprises a current-feedback circuit responsive to the sense signal, a voltage-feedback circuit responsive to the charging voltage of the battery, and a pass control circuit that regulates charging current by controlling a pass circuit, e.g., a pass transistor, responsive to a first feedback signal from the current-feedback circuit or to a second feedback signal from the voltage-feedback circuit. Note that these first and second feedback signals may be voltage-mode or current-mode signals.
An exemplary pass control circuit includes a digital switchover circuit that provides mode control switching of the battery charging circuit. In a constant-current (CC) charging mode, the pass control circuit controls the pass circuit responsive to the feedback signal from the current-feedback circuit. In this mode, the battery charging circuit regulates the charging current to a desired magnitude, which may be adjusted by changing a constant-current reference signal, for example. With that approach, the reference signal may be set for the desired xe2x80x9cC ratexe2x80x9d charging magnitude until the battery voltage reaches its xe2x80x9cfloat voltagexe2x80x9d level or some other defined threshold. Subsequently, the reference signal may be adjusted to regulate to a much lower current, such as zero, or even a negative battery current.
In any case, during the CC charging mode, the digital switchover circuit monitors the feedback signal from the voltage-feedback circuit to detect when the charging voltage reaches a defined voltage level. At that point, a mode control circuit within the digital switchover circuit switches from driving the pass control circuit with the current feedback signal to driving it with the voltage feedback signal. In other words, the exemplary battery charging circuit switches from the CC charging mode to a constant-voltage (CV) charging mode, in which the pass control circuit maintains a fixed voltage on the battery by regulating the charging voltage on the battery responsive to the feedback signal from the voltage-feedback circuit.
Complementing the switchover operation, the digital switchover circuit also switches from monitoring from the voltage feedback signal to monitoring the current feedback signal. Thus, an exemplary digital switchover circuit monitors the current feedback signal while driving the pass control circuit with the voltage feedback signal, and monitors the voltage feedback signal while driving the pass control circuit with the current feedback signal. In other words, when the battery charging circuit is in the CC charging mode, it monitors the battery voltage to determine when to change to CV mode, and when the battery charging circuit is in CV mode, it monitors the battery charging current to determine whether it should change back to CC mode regulation.
By configuring the digital switchover circuit to have common input circuitry that is shared between the constant-current and constant-voltage feedback circuits, the same circuitry is used for both the charging mode comparison control function and the charging regulation control function. One advantage of using the same pass control circuit in the charging mode comparison function that determines the charging mode and in the linear control function that sets the charging output is that any circuit offsets are common to both the comparison and control functions. Such commonality ensures that no control discontinuities arise when the battery charging circuit switches from CC charging mode to CV charging mode or vice versa.
Hysteresis may be included in the mode comparator used in the switchover circuit to prevent xe2x80x9cchatterxe2x80x9d at the switchover point. By providing a hysteresis signal that forces the feedback to overdrive the reference signal by a fixed amount before entering a given mode of operation, chatter is eliminated. This hysteresis technique safely overdrives the output voltage of the charger when maximum current is being delivered into the battery.
As noted above, the sense circuit""s wide dynamic range allows it to continue providing accurate charging current sensing even as the charging current begins falling off after the battery charging circuit transitions to the CV charging mode. Thus, in one embodiment, the charging control circuit continues monitoring the charging current via the sense circuit to detect an end-of-charge condition of the battery. Such monitoring may be incorporated into the digital switchover circuit or may be implemented elsewhere. For example, an exemplary charging control circuit includes a controller, such as a logic control circuit or state machine with appropriate signal interfaces to the current-feedback and voltage-feedback circuits.
With such embodiments, the controller detects the end-of-charge condition and adjusts a current-feedback reference signal to cause the charging control circuit to regulate the charging current for a zero, or even negative current level. Here, negative denotes a current from the battery, i.e., the battery sourcing current rather than sinking current. This zero-current regulation is in contrast to conventional chargers, which simply disconnect the battery after end-of-charge for safe isolation. Thus, the present invention""s ability to accurately detect even very low charging currents permits reliable end-of-charge detection and obviates the need for isolation switching of the battery.
Those skilled in the art will appreciate other features and advantages of the present invention upon reading the following detailed description. However, it should be understood that the present invention is not limited by the following exemplary details, nor is it limited by the accompanying drawings.